Why third party certification required .is third party certification audit requirement OR insurance requirement?

third party certification are required for inspecting the material qualaity and quantities provided by the vendor to the client 

Third party is insurance requirement and included in the coverage in may types of insurance such as :-

1) Motor vehicle insurance .

2) fire insurance .

3) engineering insurance .e.g contractors all risks insurance , and boilers explosions .

4) liability insurance . e.g public liability and product liability

5) energy insurance . e.g oil , gas , and refinery insurance .

6) aviation insurance .

I hope you will find my answer as explained above useful to you.

Best wishes . 

Apart from audit and insurance requirement, mainly third party certification is required for plant safety.